Conjunctivitis can be caused by infection from bacteria, viruses or other organisms, and also by allergy or inflammation.
Viral conjunctivitis tends to cause a watery red eye and can last for two to three weeks even with the correct treatment. In most cases viral conjunctivitis does not affect your vision but rarely you might notice your vision becomes blurry or you may see glare when looking at lights. This is due to an inflammatory reaction causing small white dots on the cornea, the transparent window at the front of the eye. These usually fade with time, but it can take a few weeks or even months.
Bacterial conjunctivitis is more likely to cause a red eye with a sticky yellow discharge.